Wire and foil elements disposed in substantially sinuous planar grid patterns have been widely used for sensing surface conditions such as strain, and have conventionally been applied to the surfaces under investigation by bonding cements and the like. In the case of highly miniaturized and exceedingly thin grids of foil, particularly, manufacturing and handling considerations have dictated that the metal foil grid be supported upon an insulating backing sheet, the latter in turn being adhered to the measurement-specimen surface by way of bonding cement. Such cements are known in a variety of compositions, and their use for the aforesaid bonding entails much care if the gage installation is to be reliable. Even at best, however, the gage responses to strain or to other influences, necessarily reflect unavoidable disturbing characteristics of both the backing and the cement, each of which is of material different from that of the other. An especially troublesome consequence is that of so-called gage "creep," a term which applies to slip of the gage relative to the surface to which it is cemented, and which involves apparent relaxation of the gage due to such factors as slip, softening or yielding of the cement, and/or like disturbances of the backing sheet, and/or similar disturbances at interfacing between the backing sheet and cement or between the cement and specimen surface or between the backing sheet and material of the gage grid. Creep is of course not conducive to accuracy, and represents a significant drawback inasmuch as the gages are intended to respond very precisely for the well-known purpose of enabling accurate measurements of elastic strain resulting from loading stresses of elements such as beams, columns, rings, diaphragms and other load or force sensing members. In addition, the usual cementing can involve difficulties in control of cement thickness, and problems with punching of the gage grid by particles found in thick paste cements, and unwanted overall of the installation, and resulting deficiencies in hysteresis, strain sensitivity, and linearity characteristics.
Related problems are advantageously minimized according to the present invention by eliminating the usual cementing altogether, and by instead bonding the gage to a specimen surface via a single thin layer of resin cured in situ in bonded relationship both to the surface and to the foil elements. In the latter connection, an improved installation and locking of the gage elements occurs because of partial embedments of the elements into the resin. Prior to installation, the gage is pre-packaged in a form convenient for handling and installation, as the result of unique processing.